The present invention relates to a double-side polishing apparatus, more precisely relates to a double-side polishing apparatus capable of measuring a thickness of a wafer while polishing the wafer.
A conventional double-side polishing apparatus for polishing both faces of a wafer comprises: a lower polishing plate whose upper face acts as a polishing face; an upper polishing plate whose lower face acts as a polishing face; a frame holding the upper polishing plate above the lower polishing plate, the frame vertically moving the upper polishing plate; a carrier being provided between the lower polishing plate and the upper polishing plate, the carrier having a through-hole, in which the wafer is held; a plate driving unit for rotating the lower polishing plate and the upper polishing plate about their axes; a carrier driving unit for rotating the carrier; and a slurry supply unit. The lower polishing plate, the upper polishing plate and the carrier are rotated with supplying slurry to the lower polishing plate so as to polish the both faces (the lower face and the upper face) of the wafer with the both polishing plates.
These days, polishing accuracy (thickness) of wafers must be higher and higher.
In a conventional double-side polishing method, firstly a polishing rate is measured by polishing a sample wafer or wafers. Next, a required time for polishing an object wafer until reaching a prescribed thickness at the measured polishing rate is calculated, and then the object wafer is polished for the calculated required time. However, the polishing rate is varied by some conditions, e.g., a surface condition of a polishing cloth, so a thickness of wafers of one batch is different from that of other batches. This problem can be solved by calculating the polishing rate of a sample wafer for each batch, but it takes a long time and it is inefficient.
To solve the problem, methods for measuring a thickness of a wafer during a polishing process have been proposed.
In Japanese Patent Gazette No. 7-52032, transparent plates are fitted to some of through-holes bored in a lower polishing plate, and a light-reflecting condition of a polished surface of a wafer is continuously monitored while polishing the wafer so as to detect completion of a film polishing process.
In Japanese Patent Gazette No. 2005-19920, an optical measuring equipment is provided on a polishing plate, which acts as a rotating section, with an optical fiber rotary joint, and a thickness of a wafer is measured through a transparent window of an upper polishing plate.
The wafer thickness measuring equipments of Japanese Patent Gazette No. 7-52032 and Japanese Patent Gazette No. 2005-19920 are shown in one drawing of FIG. 5.
In FIG. 5, a symbol 100 stands for a lower polishing plate; a symbol 101 stands for a motor for driving the lower polishing plate 100; and a symbol 102 stands for a bearing for supporting the lower polishing plate 100. A symbol 103 stands for an upper polishing plate, which is connected to a suspended plate 105 by connecting pillars 104; a symbol 106 stands for a driving section for driving the upper polishing plate 103; and a symbol 107 stands for a motor for driving the upper polishing plate 103. A symbol 108 stands for a slurry supply pipe; a symbol 109 stands for a ring-shaped conduit; and a symbol 110 stands for a slurry supply tube.
The thickness measuring equipment 111 (disclosed in Japanese Patent Gazette No. 7-52032) is provided on the lower polishing plate 100 side and emits a measuring light 113 through a transparent window 112 of the lower polishing plate 100 so as to measure a thickness of the wafer W.
The other thickness measuring equipment 114 (disclosed in Japanese Patent Gazette No. 2005-19920) is provided on the upper polishing plate 103 side, emits a measuring light 113 toward the wafer W through a transparent window 115 of the upper polishing plate 103 and introduces a reflected light to outside via a fiber cable 116, which is passed through a rotary shaft of the upper polishing plate 103, and an optical fiber rotary joint 117 so as to measure the thickness of the wafer W.
However, the above described conventional technologies have following problems.
In Japanese Patent Gazette No. 7-52032, a large ring-shaped bearing 102, which supports the lower polishing plate 100, is provided on the lower polishing plate 100 side, and the bearing 102 supports a center part of the wafer W so as to uniformly apply a polishing load to the wafer W and reduce vibration and axial runout. With this structure, the transparent window 112 must be provided in the vicinity of an outer edge of the lower polishing plate 100. Therefore, only the thickness of the outer part of the wafer W can be measured, but the thickness of the center part thereof cannot be measured.
In Japanese Patent Gazette No. 2005-19920, the thickness measuring equipment 114 including a light-receiving sensor is directly fixed on the upper polishing plate 103. With this structure, the sensor will be badly influenced by rotation and vibration of the upper polishing plate 103, so sensed data will be varied and reliability of the thickness measuring equipment 114 will be lowered. Further, a halogen light is used as a light source of an optical sensor, so a focal point of the light must be widened. Therefore, a distance to the wafer W must be about 100 mm or less.